Allah, rendering its doer out of the fold of Islam if he is a Muslim.
Tawhid, Islam, faith or salvation can only be achieved by seeking Allah (Exalted be He) alone with worship and believing that He is the Possessor of the kingdom and the One Who manages the affairs of His creatures; He is Perfect in His Self, Attributes, Names and Actions; He is the Lord of the universe and there is no partner with Him or anyone to prevent His Judgment.
This is Tawhid which is the religion of all the prophets and the meaning of Allah's Saying, You (Alone) we worship, and You (Alone) we ask for help (for each and everything).
The Ayah (Verse) means that (Allah) is the One Who we believe in His Oneness, obey, hope and fear. Commenting on the Ayah, Ibn `Abbas (May Allah be pleased with them) said, "We worship, hope and fear You Alone."
We seek Allah's Help to help us obey Him in all our affairs. Worship is to believe in the Oneness of Allah, be sincerely devoted to Him in fulfilling His Commands and abandoning His prohibitions; to have perfect belief that He is worthy of worship, the Lord of the Worlds, the Owner of everything, the Creator of everything and Perfect in His Self, Names, Attributes and Actions, and that He has no deficiency, default or partner in any of this (May He be Praised Exalted). For Him is Absolute Perfection in everything (May He be Glorified and Exalted).
Thus, it is evident that a Muslim must believe in all the Messengers and all that was revealed to them, including Prophet Muhammad (peace be upon him). However, if someone devotes their worship to Allah (Exalted be He), believes in His Messengers (peace be upon them) and especially Prophet Muhammad (peace be upon him), follows His Shari`ah (Islamic Law), stands firm in the teachings of Islam but in the meantime commits one or more things that nullify Islam, their worship is nullified and their deeds will not benefit them.
For example, if someone believes in Prophet Muhammad (peace be upon him) and his whole mission and follows his Shari`ah, but says: 'Musaylamah, who rebelled and massed people in the battle of Yamama against the Muslims and was fought by the Companions during the Caliphate of Abu Bakr AlSiddiq, is a messenger like Muhammad (peace be upon him),' they will be disbelievers and their faith will become null and void. Likewise, their good deeds will be nullified, even if they used to offer Salah (prayer) during the night and fast during the day.
This is because they commit one of the violations that nullify Islam, namely believing in Musaylimah AlKadhdhab i.e. the Liar. This falsehood includes belying Allah (May He be Glorified and Exalted) Why says: Muhammad (ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭﺳﻠﻢ) is not the father of any of your men, but he is the Messenger of Allâh and the last (end) of the Prophets.
Moreover, this includes belying the Messenger of Allah (peace be upon him) who said in Mutawatir Hadiths (Hadiths reported by a significant number of narrators throughout the chain of narration, whose agreement upon a lie is impossible) that Prophet Muhammad (peace be upon him) is the Last Prophet, so, there is no prophet after him.
Likewise, if someone offers Sawm (fasting) during the day and Salah during the night, observes different acts of worship, seeks Allah Alone with their worship and follows Allah's Messenger (peace be upon him) then once they dedicate some of their worship to someone or something other than Allah, such as offering certain worship for a prophet, a Waliy (a pious person), an idol, the sun, the moon or a star e.g. through supplication, seeking help and victory over enemies, all their works are in vain until they repent to Allah (Exalted be He).
Allah (Exalted be He) says, But if they had joined in worship others with Allâh, all that they used to do would have been of no benefit to them. and, And indeed it has been revealed to you (O Muhammad ﺻﻠﻰ ﺍﷲ ﻋﻠﻴﻪ ﻭﺳﻠﻢ), as it was to those (Allâh's Messengers) before you: "If you join others in worship with Allâh, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers."
Similarly, if someone believes in all Allah's message but denies the prohibition of adultery, sodomy, or intoxicating beverages, they will be disbelievers even if they do every good deed, because allowing forbidden matters that are known to everyone renders the person outside the fold of Islam. Thus, their good deeds and Tawhid will be useless in this case according to the agreement of all Muslim scholars.
Equally, if someone said that Nuh, Hud, Salih, Ibrahim, Isma`il, or any other prophet is not a prophet, they become nonMuslims and all their good deeds become invalid, for they belie Allah (May He be Praised) concerning what He mentioned about these prophets (peace be upon them).
Likewise, deeming as unlawful what Allah (Exalted be He) regards as lawful in spite of having Tawhid, sincerity and faith in the Messengers renders the person outside the fold of Islam. For example, if someone claims that they prohibit eating the meat of camels, cows, and sheep that Allah (Exalted be He) makes lawful and is agreed upon by scholars to be lawful, they will be regarded as apostates after showing them the evidence of the case if they are ignorant of it.
Also, if someone says that wheat, barley, or anything that is allowed to be consumed, is prohibited or says that a daughter or sister is allowed to be married to their unmarriageable kin, they become nonMuslims even if they pray and fast and offer all acts of worship, because deeming unlawful what is lawful renders the person outside the fold of Islam.
Allah (Exalted be He) says, But if they had joined in worship others with Allâh, all that they used to do would have been of no benefit to them.
Unfortunately, we are living at a time when ignorance and lack of knowledge are running rampant. People have evergrowing interest in sciences other than religious sciences and become occupied with worldly matters so that their awareness of Allah (Exalted be He) and His religion become weak. These things took them away from religious knowledge. You may find most scientific talks are concentrated on financial subjects whereas comprehending matters of religion and learning Shari`ah and Tawhid are neglected and abandoned.
Therefore, one should pay attention to this matter and be interested in the Book of Allah and the Sunnah of His Messenger (peace be upon him). One should study and consider them in order to understand the meaning of Tawhid, Iman, and Shirk and to be well acquainted with religious knowledge. Understanding the texts of the Qur'an and Sunnah makes one realize the ways to Paradise and Salvation from Hellfire. This is especially true when a person is keen to attend religious sessions and study under pious scholars in order to gain knowledge and spread useful knowledge to others and be aware of the religion.